One of the problems in the electronic components field involves the measurement of weak signal components produced by an electronic device in the presence of obscuring background noise. A large signal input into a device will normally override the noise component in a given device. With a small signal input, the background noise, inherent in the testing and measurement process, can totally block detection of the signal transmitted through the device. Once the signal which the device is intended to produce attains a lesser magnitude than the noise existing in the test system, the signal appears not to exist.
In a general sense, noise and interference are used to describe any unwanted or undesirable signals in communication channels and systems. Since in many cases these signals are random or unpredictable, the characteristics of random processes can be used to defeat noise effects. Since random noise can have both a positive and negative effect upon a system, it may be expected that the magnitude and occurrence of the positive effects equal the magnitude and occurrence of the negative effects, causing the net effect of the noise to disappear. Once the magnitude of the signal to be detected falls beneath the magnitude of the randomly occurring noise, the problem of separating the signal from the noise must be solved in order to see how well the device handles the small signal in a test or to acquire the desired signal in an application.
The transfer function characterizes an electronic unit; with it the output resulting from a specified input may be computed. Usually, the transfer function of an ideal device is a mathematical operator which operates on a given input to produce an output proportional to that input. For real devices, unfortunately, the transfer function can change with respect to the intensity of the input signal. The transfer function must be determined over the full range of useful input amplitudes. To further complicate matters, small input amplitudes and the correspondingly small output amplitudes they produce are often enveloped in surrounding noise. This noise is characteristic of almost any electronic environment. Where the output signal is buried in noise, it is extremely difficult to discern the output, due to a controlled input, from the noise. In consideration of both these purposes, namely, the characterization of transfer function and the measurement of the small signal output in the presence of noise, it is desirable to be able to test a particular device for faithful operation under small signal conditions and to test large numbers of devices to find the characteristic transfer function under conditions of changing input signal strength. This is especially useful for oPtical and other devices in which nonlinearities often occur.
One solution to this problem has been the use of the lock-in amplifier. Commercially available lock-in amplifiers can measure weak input components in the presence of obscuring background noise, but the operating frequency range is generally limited to from 0.1 hertz to 200,000 hertz. The output of the lock-in amplifier is a phase-sensitive direct voltage which is proportional to the input signal. Basically, the lock-in amplifier detects weak signals by the process of synchronous demodulation. Extraneous signals which are not synchronized to the reference are rejected.
Lock-in amplifiers usually have a low-noise pre-amplifier and a post-amplifier, with a filter sandwiched in between. This filter, often referred to as a pre-detection filter, is for the purpose of reducing the possibility that the lock-in amplifier will overload during severe noise conditions. Pre-filtering can distort the signal sought to be measured, depending upon the frequency characteristics of the input signal and the frequency characteristics of the pre-filter. It is recognized that the pre-filter also has a transfer function which, in effect, "couples" with the transfer function of the device sought to be measured. For example, since pre-filtering is for the purpose of eliminating the higher harmonics of the fundamental carrier frequency, if the transfer functions of the device to be measured are to operate on the higher frequencies as well, the high frequency portion of the output will be attenuated. This would have the effect of masking the very distortions which are attempted to be measured.
Also, since filters are frequency specific, when the signal frequency is changed, the frequency of the pre-filter and the lock-in amplifier must be changed. Also, since a lock-in amplifier is an analog device, time must be expended during a test to allow sufficient time for the lock-in amplifier to stabilize. This limitation severely extends the total time required for making measurements.
Furthermore, lock-in amplifiers are bulky and expensive. If several parallel measurements of the same type are to be made on a single multi-channel device, one such lock-in amplifier must be supplied for each "path" or channel within the device, and each lock-in amplifier must be set up for its respective measurement frequency and then allowed to reach equilibrium before useful data may then be taken.
The output of the lock-in amplifier is also dependent upon the phase difference between the signal to be measured and the reference driver. The reference driver defines the length of the time period of the fundamental frequency of the signal input.
The final section of the lock-in amplifier, namely the low pass filtering DC amplifier combination, follows the synchronous demodulation; it amplifies the DC component of the signal and attenuates the AC components of the signal. The attenuation is dependent upon the frequency of the input and reference signals. Thus, with a lock-in amplifier, device attenuation response will be skewed depending upon the frequency used to test the device.